Police have arrested the half-brother of senior Conservative MP John Whittingdale as part of an inquiry linked to allegations that a VIP paedophile ring abused children in care in the 1980s.

Charles Napier, 65, was arrested at his Dorset home by officers working on a new strand of an inquiry set up to look into alleged abuse at the Elm Guest House in south-west London.

Police spent 40 minutes inside the house in Sherborne before leaving with Mr Napier, according to the investigative website Exaro.

Mr Whittingdale, 53, chairman of the Commons Culture, Media and Sport Committee, said of his half-brother’s arrest: “If allegations have been made, of course I accept the police should investigate this.”

Scotland Yard set up Operation Fairbank to assess early evidence after claims by the Labour MP Tom Watson that information about a paedophile ring had not been pursued by police. The MP had claimed that a file of evidence that was used to convict a man called Peter Righton of importing child pornography in 1992 contained “clear intelligence” of a sex abuse gang.

A full-scale criminal investigation, Operation Fernbridge, was launched in January. Scotland Yard revealed earlier this month that officers were pursuing more than 300 lines of inquiry and a list of several dozen potential victims was being drawn up. The allegations included ones of multiple assaults on individuals.

The number of leads being pursued indicated that the inquiry was much bigger than previously acknowledged and the Metropolitan Police said that the arrest was made as part of Operation Cayacos, a previously unknown strand of the inquiry. It is not known whether the allegation against Mr Napier relates to an underage victim.

The Elm Guest House became known in the late-1970s as a meeting place for gay men still stigmatised in a country where homosexuality had been legalised a decade earlier and the age of consent for gay males was 21. Operated by German-born manager, Carole Kasir, it was close to Barnes Common, a popular cruising spot for gay men, and was allegedly used by rent boys as a place to bring clients.

But officers are investigating material alleging that boys from the nearby children’s home in the care of the local authority were abused at the guest house, which was allegedly frequented by public figures including politicians, judges and pop stars. Mr Napier is the third person to be arrested as part of the wider Fernbridge inquiry.